I installed the Flash Player plugin, and videos play, but
without sound. So I tried to run CCleaner (a suggested fix for this
problem). However, CCleaner does not list Flash Player among the
Multimedia selections. Is the plugin not the full version? Does
installing only the plugin (when no prior "full" version is
installed) result in only a partial version? Seems unlikely, but
something seems incomplete about my install.

"Is the plugin not the full version?" - a plugin is an
additive install (no matter what program it's for), so there is no
such thing as a "full version" plugin. Are your audio drivers up to
date? What version of the Flash player plugin did you install?
Where did you get the file?

Oh, the version of the plugin is 9.0.124.0. I just can't find
a full version to install. The plugin plays video (e.g., YouTube)
visuals fine; it just doesn't play the audio. I'm hoping installing
a full version will solve this.